[American Institute of Aeronautics and Astronautics 37th Aerospace Sciences Meeting and Exhibit - Reno,NV,U.S.A. (11 January 1999 - 14 January 1999)] 37th Aerospace Sciences Meeting

(c)l999 American Institute of Aeronautics & Astronautics

A9946009

ATAA 99-0009 Variable Surface Area Paraglider Zachary C. Hoisington California Polytechnic State University San Luis Obispo

37th AIAA Aerospace Sciences Meeting an Exhibit

January 11-14, 1999 / Reno, NV

For permission to copy or relpublish, contact the American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, VA 20191


VARIABLE SURFACE AREA PARAGLIDER

Zachary C. Hoisington* Aeronautical Engineering Department California Polytechnic State University

San Luis Obispo

Abstract

An attempt was made to improve the performance and stability of a paraglider. A conventional paraglider accelerates when the angle of attack is reduced, which makes the wing more prone to collapse and greatly reduces the lift-to-drag ratio. To improve the performance and safety, a device was constructed that allows the surface area of a paraglider to change while in flight. Experiments were conducted dealing with the structure and flight behavior in order to construct the new type of speed system. Based on flight test results, higher speeds were reached with greater glide ratios and improved stability over conventional paragliders. Paragliders, sky diving parachutes, and other types of parafoils may benefit from this research.

AR

b

LID

s

Nomendature

aspect ra.tio, E b2 I S

wing span

lift-to-drag ratio, same as glide ratio

reference, taken as wing planform area

* Undergraduate student. RIAA Student Member.

brakes

glide ratio

parafoil

paraglider

point

riser

sink rate

soaring

speed bar

steering toggles

trim speed

Terminolow

main controls for a paraglider, deflects the trailing edge same as steering toggles

ratio of the distance traveled forward to the distance dropped, same as LID

a ram-air parachute with an open leading edge and closed trailing edge

a specific type of parafoil, used for foot-launched soaring flight

one unit in a glide ratio comparison (e.g. there is a 1 point difference between a 7: 1 and 6: 1 glide ratio)

straps between the lines of a paraglider and the pilot

descent rate of an aircraft relative to the air mass

to be able to remain aloft without descending (requires rising air)

foot-bar that pitches the leading edge of a paraglider down for acceleration

main control lines for a paraglider, deflect the trailing edge, same as brakes

the hands-off flying speed of the paraglider, usually the speed of maximum L / D

Copyright 0 1998 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

American Institute of Aeronautics and Astronautics


Introduction When the wing loading on a gliding aircraft is

A paraglider is an aircraft made of fabric and suspension lines, as shown in Fig. 1. A small area near

Paraglider Figure 1

the leading edge is open to give the wing internal pressure allowing it to keep its shape. A paraglider is foot-launched, and does not require free-fall to initiate flight. In the past ten years, there have been many design improvements with the paraglider. During this time, the performance and stability have increased remarkably.

Various types of parafoils are used for sky diving, paragliding, kites, and spacecraft recovery. There is one common bond between nearly all types of parafoils used today: they have a very limited speed range. Along with other non-motorized aircraft, acceleration of the parafoil is achieved by a reduction in the angle of attack. When speeding up a parafoil, the L ! D decreases rapidly, and the wing becomes more prone to collapse, as shown in Fig. 2. When the

Positive Angie of Attack.

Negative Angle of Attack

Figure 2

angle of attack is positive, as shown on the left, a parafoil will keep its shape. When the angle of attack is negative, as shown on the right, the wing will collapse downward from the force of the air.

increased, there is minimal change in the L / D, and higher speeds can be reached. This is because the lift and drag forces change proportionally to each other. If an aircraft could instantly scale itself to the desired size in flight, the maximum L /D could be reached over a variety of airspeeds. This would allow the maximum possible range regardless of the direction and speed of the wind.

One current type of aircraft employs the capabilities described above: they are birds. When seagulls need to climb in rising air, their wings are tilly extended for maximum lift. When they want to go fast, they tuck in their wings and fly on the tips, allowing greater stability and L /D than they could achieve at that speed with their wings spread out.

A variable surface area system was developed on a paraglider to improve L / D over a wide range of speeds. A system was developed that squeezes the center of a paraglider together while in flight. Many prototypes were tried until significant performance and stability benefits resulted.

Procedure and Apparatus

Prototypes were tested by following the general procedure below.

Stev 1 The modified wing was tested as a kite in

winds between 10 and 20 mph. This simulated flight testing allowed the glider to support nearly all of the weight of the pilot without becoming airborne. Once the paraglider was held up overhead by the wind, the brakes were deflected through a range of motion similar to that experienced in flight. The turning characteristics, stall speed, and leading edge stability could all be estimated while kiting. This served as a good airworthiness check and familiarized the pilot with the handling of the gmer before taking flight.

Stev 2 The modified wing was then taken to a large

sandy slope with a strong up-slope wind. Low-level flights were done where the total time in the air was only a few seconds and the maximum altitude was less than one meter. This testing was useful as a final verification that the paraglider was ready for a high altitude flight.

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Steu 3 A high altitude flight was done in smooth air.

These tests were primarily performance tests measuring airspeed and descent rate. The flights were done from large mountains &mater than 500 meters difference between launch and landing). This allowed enough altitude for a reserve parachute deployment. A special protective harness was worn as well, as shown in Fig. 3.

This allowed data to be taken when there was strong wind. The experimental paraglider was flown beside other paragliders and glide ratio and speed comparisons were made.

For performance measurements, the airspeed of the paraglider was adjusted by pressing on the speed bar with the feet (Fig. 5).

Protective Harness Figure 3

Instrumentation was used to record the airspeed and descent rate (see Fig. 4). A GPS, barograph, variometer, and airspeed indicator were all used.

Variomctcr G.P.S Figure 4

Because the barograph, GPS and variometer are instruments which record information with reference to the ground, the air had to be very still for accurate data. This required flying very early in the morning or late in the evening. Even if there was no detectable wind on the takeoff area, there was often too much vertical movement of the air (greater than .2 m/s) for accurate data acquisition. Over ten flights were completed and an average of the data was used.

Data was also gathered by comparing the performance to other paragliders while formation flying.

Speed Bar Riser Figure 5

The speed bar pulls down on the front riser, which is connected to the leading edge of the paraglider. When taking performance data, the speed bar was pushed a percentage of its maximum travel and then held constant. After the paraglider stabilized at this flight speed, the data was taken from the flight instruments. This was done for the paraglider with the surface area reduction system engaged and t-Idly released.

Results and Discussion

Performance Prediction To simulate the maximum benefits of the

surface area reduction device, paragliders of different size were flown and the performance was measured. Measuring the glide ratio at different speeds was the primary consideration. According to Pagen,’ when the wing loading is altered on a paraglider there is no change in glide ratio. To further test this, different sizes of the same type of paraglider were flown many times from a mountain in smooth air. When a paraglider was flown that was 20% smaller, the change in glide ratio wasn’t measurable. When the performance was compared to a wing that was effectively 40% smaller, the L /D decreased from an original 7.5 to 7.3. The speed increase, however, was an impressive 20% greater with the smaller glider. When the speed bar was pressed fully on the Relax paraglider, the L / D decreased from 7.87 to 4.98. At this descent rate there

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was only 13% increase in forward speed from trim speed.

From the above results, reduction in the surface area of a wing shows promising hope of being a superior accelerator system, by holding nearly the same glide ratio while increasing speed. An ideal surface area reduction device would be able to change the size of the wing while not increasing the drag with exposed folds of fabric. The main goal of the project was to reduce the surface area as cleanly as possible. The maximum glide ratio that the paraglider can achieve without a speed system was measured to be 7.87: 1. This is comparable to the results from the glide ratio measured at the 1998 StubaiCup,’ where the same model achieved a 7.53: 1. With the system engaged, the glide ratio that hopefully would be achieved would be as close to this as possible.

Descriution of Desien The system itself consists primarily of lines

and pulleys that are added to a paraglider (see Fig. 6). The system is controlled by lines [I] and [2], that travel up from the point where the pilot is positioned. The lines travel up to the parafoil and split into two lines [3], and then run through pulleys that are mounted on the surface of the parafoil [4]. The line then runs across the region of the wing that is squeezed together and through a pulley [5]. The line then runs to the other side of the squeezed region and through another pulley [6], and then back to [5]. This gives a 3:l mechanical advantage. The pulleys at [4], and others like it are called anchoring pulleys. They support nearly all of the downward force that is applied to the wing from [l] and PI.

a

G- Area Pulled --+ Together

Anchoring Pulley

All of the pulleys attached to the wing were tied to the existing loops on the wing at the line attachment points. The lines that run between [5] and [6] and others like it, such as the lines between [7] and [8], are positioned so that that there is minimal interference with the suspension lines of the parafoil. Lines [l] and [2] are positioned so that if they are pulled evenly, the wing tips of the parafoil won’t twist independently of each other.

One batten [9] was placed so that the local region that is pulled together will have less wrinkling. The batten was not necessary for the system to be functional, but it improved the performance.

Figure 7 and Figure 8 show what the paraglider looks like with and without the system engaged during a ground test. The system can be pulled in and then released back to the normal configuration in a matter of seconds while in flight.

Structural InteariiG The system reduced the number of lines that

were suspending the pilot by 35%. The total loss in line strength (lines in the center of the wing no longer support any load) drops by about 40%. The Relax paraglider used was tested to 1400 Kg before there was complete failure of all of the suspension lines from the DHV testing.3 For the weight of the test pilot and equipment (80 KG), the glider was theoretically still capable of 10.5 G’s All of the testing was done in level flight, so there was little worry of exceeding this.

I I

To Pilot Under-Surface of Wing To Pilot

1;3 Figure 6 0



Figure 9. Since the parafoil has no rigid structure, it will collapse downward if there is too much force applied to any part of the wing.

The most apparent danger of applying a downward force to the wing in the anchoring region was that collapses in this area would happen more frequently and not re-inflate as well. The anchoring

System Not Engaged Figure 7

System Engaged Figure 8

To consider the difference in strength of the wing as the angle of attack is changed, an experiment was performed to measure the relative loads on the lines, A strain gauge was attached to different lines and the forces were measured with the speed bar pushed to different settings. It was found, however, that there was no significant change in the loads. This can be attributed to the change in the lift distribution of the wing with the change in angle of attack.

Anchoring Reaion The main lines that pull the wing together are

anchored by an area towards the tips of the glider. The anchoring area is always under a horizontal and downward force from the system as can be seen on

Under-Surface View From Trailing Edge Figure 9

region that was chosen was put aft of the center of pressure of the wing. This is because if that area was deflected downward from the force applied by the system, it would be less likely for that part of the wing to fold down. If that region was pulled down by a force, the average angle of attack of the wing in that region would increase, resulting in more lift in that area. The exact locations for the anchoring areas were determined experimentally by mounting additional lines directly to the surface of the wing. These different areas were pulled on in flight and the most robust area was chosen.

To reduce the force on the anchoring area, the mechanical advantage of the system of pulleys on the wing could be increased, or the anchoring area could be expanded. Having a larger anchoring area required attaching lines to the wing that suspended the two pulleys beneath the glider, as seen in Fig. 10.

Under-Surface of Simple System Figure 10



This system worked very well, but during the ground testing, the system would occasionally tangle with the main suspension lines. Such a tangle could cause a severe problem with the stability of the paraglider and lock the system permanently while in flight.

Collapse Stability If a collapse were to occur from turbulence, the

system had to allow the wing to inflate without interfering. From Pagen,’ the majority of the span-wise tension in a paraglider is created from the wing tips (stabilizers) of the paraglider. This force is directly proportional to the force that must be applied to the anchoring area to keep the wing pulled together. The force that the wing applies to the anchoring area is proportional to the force applied to the anchoring region. When the wing tips collapse, the tension in the glider decreases, so the force required to keep the system engaged drops as well. Therefore, the system never had enough force to keep a collapse from inflating. All collapses that occurred during the testing inflated rapidly with no interference from the system.

Line Storage When the system was engaged, 8.5 meters of

line had to be pulled in by the pilot. Storing this line was a big complication. For most of the testing the line was often just pulled in and allowed to trail behind the pilot in the wind. This technique had a big danger of an entanglement. A foot stirrup was made that allows the line to be stored in a neat fashion, but it required storing the lines in a series of pulleys~to displace 8.5 meters with one push from the feet. When the pulleys were exposed to sand, there was too much friction in the system to engage it without a manual assist.

Mechanical Advantage The ease of use of the system was greatly

improved with a reduction in the line displacement. To decrease the line displacement, the mechanical advantage of the squeezing mechanism had to be reduced. The limiting factor on the mechanical advantage was the deformation of the wing in the anchoring region. The first systems tried used a 5:l ratio. To achieve this, from Figure 6, there were four lines between [5] and [6]. The same was true for [7] and [8] and the other pulley sets. With this system the paraglider pulled in very easily, and there was no noticeable deformation in the anchoring region. Lower mechanical advantages were tried. With a ratio of 4:l there was still a negligible amount of deformation to the anchoring region of the wing. A ratio of 3: 1 seemed to be the limit. With a 2:l ratio, the anchoring region would occasionally merge with the center of the glider

during the kiting tests. To test this further, the system was engaged while in flight. The wing would pull together somewhat cleanly while in flight, but there was the danger that the wing would have a sudden and large deformation. With a 3:1 ratio, there was still no concern with the collapse stability, but there was a definite start in the-deformation of the wing. This was especially true at high angles of attack when the wing experienced more force pulling apart. A 3: 1 ratio was decided upon, making for 8.5 meters of line that had to be pulled in to engage the system.

Pulleys Versus Metal Rings The original prototypes used metal rings

instead of pulleys on the surface of the wing. The metal rings caused a large amount of friction in the system. When the 5:l system was used, the wing would often stick together while in flight. With the 3: 1 system, there was still an unacceptable amount of friction. In an attempt to reduce this, a braided nylon line was used instead of fishing line, but the efforts were futile. With a slow opening of the wing, it was easy for part of the system to release faster than the rest. This caused a large sweep angle in the wing, resulting in a stability problem.

Pulleys were the only solution found, but they added some major complications. If the line running through the pulley is too small, the line can jam between the wheel and the casing. This would cause the system to be locked in that position with little hope for recovery. Therefore, a thicker, braided nylon line was used, increasing the parasite drag of the system. Another problem with pulleys is that they have an unpredictable amount of friction if they get dirty. With testing at the beach, sand was always getting inside them. Nevertheless, with the pulleys, the amount of friction in the system was greatly reduced. Even when kiting the wing (span-wise tension is very light) the wing would snap open very quickly when the system was released. No problems ever occurred with pulleys jamming.

Paraglider Choice :- The paraglider used for the final version of the

system was the Pro Design, Relax (Fig. 1). Although the wing wasn’t specifically designed for the system, there were several reasons why it worked well.

With the 3:l pulley ratio, there was too much deformation of the wing at the anchoring area if the pulleys were hooked directly to the wing’s surface--on a paraglider without diagonal ribs. Diagonal ribs distribute force that is applied to the wing to three different points on the upper surface.

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Under-Surface Close-Up Figure 1 I

The Relax has diagonal ribs extending from every line attachment point on the wing. With the diagonal ribs, much more force could be applied to the wing without deformation. This allowed the anchoring pulleys to be attached directly to the surface of the wing.

The Relax has a fairly low aspect ratio for its performance class. From Hofbauer,4 the projected aspect ratio is only 3.59. With a low aspect ratio, a smaller width of the glider had to be pulled together to eliminate a proportionally greater amount of the surface area. A lower aspect wing also suffers a smaller change in the overall aspect ratio when the tips are pulled toward each other by the system.

Line Arrangement of the System One prototype of the system allowed the sweep

of the wing to change depending on the pull on the main lines (see Fig. 10). During the kiting tests, it was found that there were dangers if the sweep of the wing was changed while the system was transitioning between the closed and open position. With any forward sweep, there was a great loss in yawl stabiIity. This was later verified in a high altitude test unintentionally. With aft sweep, there was a large increase in the angle of attack of the glider, causing a dangerous reduction in airspeed.

A system that doesn’t change the sweep at any time was necessary. To do this, the main lines had to be able to engage the system even if only one of them was pulled and the other was released. This property vkould also add to the safety if there was some sort of tangle in the system. With the apparatus shown in Fig. 6, when one of the lower Lines [l] or [2] are pulled, the wing will keep the same sweep.

Reducing The Wrinkling The system that only pulled the glider together

at two points (Fig. 10) did little to improve the performance of the glider. The maximum speed was higher than what could be achieved without the system, hut the descent rate was extremely high. This most likely could be attributed to the increase in drag in the center of the glider. There was also a structural danger with this configuration, because all of the tension of the wing was held by only two locations. Turbulence in flight could have caused the wing to rip apart in the center.

More pulley sets were added to the wing to come up with the final configuration in Fig. 6. This made a major improvement in the descent rate at maximum speed. The batten was very helpful in reducing the wrinkles as well. Without it, the trailing edge of the glider formed a large fold that hung down beneath the rest of the wing.

Trim Angle Without touching the speed bar, engaging the

system did not increase the airspeed of the paraglider. The reason for this was that the angle of incidence increased when the wing was pulled together. This could be attributed to a couple of things. One reason is that the parafoil has washout. The tips have 3 degrees higher angle of incidence than the center of the wing. When the center of the wing is pulled together, the average angle of incidence of the wing increased. The second reason why the angle of incidence was increasing was that the induced drag was increasing. With the increase in induced drag, the canopy was rotating behind the pilot more than it does in normal flight.

To compensate for this problem, an adjustable strap was added to the most aft riser. When the system was engaged, the glider was trimmed faster by releasing the adjustment manually. This had the same effect of permanently pushing part-way out on the speed bar. The amount that this riser was released was determined from flight tests comparing the control input required to reach stall speed.

Top Ballooning When the system was engaged, a large area in

the center of the wing raised up higher than the rest of the upper-surface, as shown in Fig 12.



Forward Speed

(meters/second)

Without System System Fully Engaged Engaged

(meters/second) (meters/second)

Upper Surface Figure 12

The hump is formed because of the line cascading. Since the lines branch off into many different lines from the pilot to the wing, the middle lines become longer (taking a shorter path) when the system is engaged. To fix this, lines were added that run directly from the pilot to the wing. This system, however, formed many small wrinkles rather than one large hump on the upper surface. Having one large hump rather than many smaller ones improved the glide ratio. Keeping the area that was being squeezed together mostly inflated also allows better air travel across- the span inside of the parafoil. That would be a big advantage if there was a large collapse on one side of the glider.

Flight Test Data Table 1

5 6 yrmrd Speed (M&m/Second)

E 8 IO 11 12 13

Another way to keep the paraglider from l Wthaut Syztm Engaged

forming a hump on the upper surface is to mount eyelets on the wing for the system’s lines to run through. These can be seen on the leading edge of the wing on Figure Il. The drawback from the eyelets was that they added to the force required to engage the system, so they weren’t used.

Experimental Polar Figure 13

When the wing was accelerated above 10 m/s, the performance of the wing with reduced surface area was very noticeable. The wing also reached a maximum speed of 12.52 m/s, instead of the 10.95 m/s of the wing fully opened. This is a 14.3% increase in speed, still lower then the 20% speed increase recorded from the glide tests with different sized gliders.

Flight Performance A polar was plotted from the flight test data

(the flight test data is shown in Table 1). The maximum glide ratio was measured to be-a 7.87 to 1 without the system engaged. As the wing was accelerated with the speed bar, the glide ratio decreased to 4.98 to 1, at a maximum speed of 10.95 m/s.

With the surface area reduced, the wing showed a considerably higher sink-rate at slow speeds. This was expected, and can be attributed to the decrease in aspect ratio and the extra drag from where the wing was bunched together in the center. Nevertheless, a maximum glide ratio of 6.8 to 1 was still achieved. This is only a 1.07 point drop from the 7.87 L / D that was achieved with the wing fidly open. More remarkably, at this speed, the wing without the system engaged was only getting a L / D of 5.1.

10.73 -1.76 -1.50 10.95 -2.20 -1.61

oSrjtem Fully Engaged

The increase in performance is more profound if a headwind or sinking air encounters a parafoil. If there was a 5 m/s headwind, the variable surface area paraglider would have a 12.6% superior glide ratio. If there is a 10 m/s headwind, the shrinkable glider has a 110% increase in glide ratio.

The system itself didn’t seem to hurt the performance of the paraglider without the system engaged because the performance was measured without the system attached to the wing, and no difference was noted. This was expected because the only difference wG the negligible extra parasitic drag from the lines of the system.

d



Stabilitv With the surface area reduced, the paraglider

was less prone to collapse than with the wing lily open. This is partially because the angle of attack of the wing is higher for a given airspeed. For part of the wing to collapse, it would take a greater disturbance to make the angle of attack become negative.

Since the system decreases the span of the glider, there is also added stability bonus. When the wing is in the open position, it will encounter a wider section of air. This means that on the average, there will be a bigger change in the vertical velocities of the air across the span of the glider. This will make the change in the angle of attack across the glider from turbulence more extreme, increasing the chance of the wing collapsing. This concept can be seen in Figure 14. The top paraglider has a shorter span, so it doesn’t

Stability Comparison Figure 14

encounter the sinking air that causes the lower paraglider to have a small collapse.

When engaging the system and pushing f3ly on the speed bar, the glider still felt very stable. Small collapses that were engaged manually by the pilot recovered on their own without any input. The stability was so great that fi@her acceleration is possible by using a speed bar witb greater line displacement. The system was easy to engage and release. After the pilot

released the main lines, the wing would snap to the filly open position within 2.5 seconds.

Fli& Applications From Ferrer: the current glide ratio of a high

performance (competition) paraglider compared to one that is suited for beginners is only about 1 glide point. The stability and safety of the beginner gliders, however, is far superior. The variable surface area system improves the glide ratio by nearly the same amount that the competition and beginner paragliders differ at high speeds. If a paraglider that is normally suited for beginners is equipped with the system and then flown with a lighter than normal wing loading, the performance would be very similar to that of a competition wing. With a lighter than normal wing loading, the minimum sink rate would be comparable to that of the competition gliders. For flying fast, the surface area could be reduced and a similar glide ratio to that of the competition glider could be achieved. Thus, a beginning glider equipped with the system can have the performance features of a competition glider.

If the system was added to a sky diving ram-air parachute, it could be used in an emergency if strong wind was encountered. Landing in strong winds is a big danger in both paragliding and sky diving, and the system allows for a faster and more stable descent than what is currently possible.

The system may also have an application for the recovery of spacecraft that use a parafoil. If high wind or sinking air is encountered, the system could be engaged to increase the chances of making it to the landing area. It may also add an advantage if there is strong wind on the ground so that the craft doesn’t land traveling backwards. For a paraglider, a speed bar alone works well because the pilot is able to constantly monitor the angle of attack of the paraglider and adjust for turbulence. A spacecraft, however wouldn’t have this ability, and may benefit from a surface area reduction system.



Conclusions References

A system was developed that improves the performance and stability of a parafoil. When flying faster than trim speed, the variable surface area system allows a paraglider to go faster without losing as much altitude. At this speed, the paraglider is less prone to collapse than it would be with a speed bar alone, further increasing the usable speed range of the paraglider.

When the wing loading is increased on a conventional paraglider, the glide ratio has a very minimal change while there is a notable increase in speed. The L/D dropped over 1 point with the system engaged, showing that there is room for titure development. The speed increases were only a little better than half of what theoretically could be achieved as well.

1.

2,

3.

4.

Pagen, Dennis, Walking on Air. Paragliding FliPht, lSt Ed., Dennis Pagen, PA, 1990, ppI-37-39.

Ferrer, Michael, “‘98 Stubai Cup,” Cross Countrv, Issue No. 55, Feb. 15, 1998, pp. 20-21,46-47.

DHY (Deutscher Hangegleiterverband, Schaltlacher Strasse 23, 8 184 Gmund am Tegersee, Germany.

Hofbauer, Herbert , “Owners Manual,” Pro Desia u, Vo1.1, Innsbruck, Austria, 1998, p. 17.

The final system created is not near the optimum design. With unlimited money and time, a parafoil could be specifically designed for the surface area reduction system. The possibilities are many. The wing could be built with a shorter chord at the tips so the aspect ratio with the system engaged would be the same as if the wing was fully open. The airfoil sections at the wing-tips could be optimized for speed. The internal structure at the anchoring region could be made to hold a greater force. The entire system could even be mounted inside the parafoil to reduce drag.

Stalls and spins were never performed with the system folly engaged because of the danger involved in the testing. Before the system is developed any firrther, this type of testing should be completed.

The system developed only reduced 38% of the surface area of the wing. A more aggressive system that eliminates more than half of the wing is certainly possible.


Documents

[American Institute of Aeronautics and Astronautics 37th Aerospace Sciences Meeting and Exhibit - Reno,NV,U.S.A. (11 January 1999 - 14 January 1999)] 37th Aerospace Sciences Meeting